被xx跳动大佬使劲儿蹂躏了一把，赶紧回来总结总结
本文参考红橙大神的博客
<ImageView
android:id="@+id/iv"
android:src="@mipmap/ic_test"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
iv.setImageDrawable(getResources().getDrawable(R.drawable.ic_launcher_background));
一句话就能把图片资源加载完成，凭啥啊？
1. 翻源码
首先，src是通过TypedArray获取的，点开ImageView的源码直接找对象
final TypedArray a = context.obtainStyledAttributes(
attrs, R.styleable.ImageView, defStyleAttr, defStyleRes);
final Drawable d = a.getDrawable(R.styleable.ImageView_src);
if (d != null) {
setImageDrawable(d);
}
有两个方法值得注意一下：
- getDrawable 返回Drawable对象
- setImageDrawable，这个方法是不是很眼熟
@Nullable
public Drawable getDrawableForDensity(@StyleableRes int index, int density) {
if (mRecycled) {
throw new RuntimeException("Cannot make calls to a recycled instance!");
}
final TypedValue value = mValue;
if (getValueAt(index*AssetManager.STYLE_NUM_ENTRIES, value)) {
if (value.type == TypedValue.TYPE_ATTRIBUTE) {
throw new UnsupportedOperationException(
"Failed to resolve attribute at index " + index + ": " + value);
}
if (density > 0) {
// If the density is overridden, the value in the TypedArray will not reflect this.
// Do a separate lookup of the resourceId with the density override.
mResources.getValueForDensity(value.resourceId, density, value, true);
}
return mResources.loadDrawable(value, value.resourceId, density, mTheme);
}
return null;
}
本质还是调用了mResource里面的方法.
context中的处理
首先看Context关于getresource()方法 , 最终定位到这里
private Resources getResourcesInternal() {
if (mResources == null) {
if (mOverrideConfiguration == null) {
mResources = super.getResources();
} else {
final Context resContext = createConfigurationContext(mOverrideConfiguration);
mResources = resContext.getResources();
}
}
return mResources;
}
Resource的实例由Context 的子类创建，但是无法找到mBase的具体实例。 我们现在ContextImpl中找找。
private ContextImpl(ContextImpl container, ActivityThread mainThread,
LoadedApk packageInfo, IBinder activityToken, UserHandle user, int flags,
Display display, Configuration overrideConfiguration, int createDisplayWithId) {
......
Resources resources = packageInfo.getResources(mainThread);
if (resources != null) {
// 不会走此分支，因为6.0中还不支持多屏显示，虽然已经有不少相关代码了，7.0以及正式支持多屏操作了
if (displayId != Display.DEFAULT_DISPLAY
|| overrideConfiguration != null
|| (compatInfo != null && compatInfo.applicationScale
!= resources.getCompatibilityInfo().applicationScale)) {
......
}
}
......
mResources = resources;
}
// packageInfo.getResources 方法
public Resources getResources(ActivityThread mainThread) {
// 缓存机制，如果LoadedApk中的mResources已经初始化则直接返回，
// 否则通过ActivityThread创建resources对象
if (mResources == null) {
mResources = mainThread.getTopLevelResources(mResDir, mSplitResDirs, mOverlayDirs,
mApplicationInfo.sharedLibraryFiles, Display.DEFAULT_DISPLAY, this);
}
return mResources;
}
接下来跟到ResourceManager中
Resources getTopLevelResources(
String resDir, //app资源文件夹路径,实际上是apk文件的路径,如/data/app/包名/base.apk
String[] splitResDirs,//针对一个app由多个apk组成（将原本一个apk切片为若干apk）时，每个子apk中的资源文件夹
String[] overlayDirs,
String[] libDirs, // app依赖的共享jar/apk路径
int displayId,
Configuration overrideConfiguration, CompatibilityInfo compatInfo) {
final float scale = compatInfo.applicationScale;
Configuration overrideConfigCopy = (overrideConfiguration != null)
? new Configuration(overrideConfiguration) : null;
// 以apk路径为参数创建key
ResourcesKey key = new ResourcesKey(resDir, displayId, overrideConfigCopy, scale);
Resources r;
synchronized (this) {
// Resources is app scale dependent.
if (DEBUG) Slog.w(TAG, "getTopLevelResources: " + resDir + " / " + scale);
// 如果持有弱引用, 直接复用并返回
WeakReference<Resources> wr = mActiveResources.get(key);
r = wr != null ? wr.get() : null;
if (r != null && r.getAssets().isUpToDate()) {
if (DEBUG) Slog.w(TAG, "Returning cached resources " + r + " " + resDir
+ ": appScale=" + r.getCompatibilityInfo().applicationScale
+ " key=" + key + " overrideConfig=" + overrideConfiguration);
return r;
}
}
//使用apk的路径来初始化AssetsManager
AssetManager assets = new AssetManager();
// resDir can be null if the 'android' package is creating a new Resources object.
// This is fine, since each AssetManager automatically loads the 'android' package
// already.
if (resDir != null) {
if (assets.addAssetPath(resDir) == 0) {
return null;
}
}
...
// 没有弱引用，重新创建资源
r = new Resources(assets, dm, config, compatInfo);
if (DEBUG) Slog.i(TAG, "Created app resources " + resDir + " " + r + ": "
+ r.getConfiguration() + " appScale=" + r.getCompatibilityInfo().applicationScale);
synchronized (this) {
WeakReference<Resources> wr = mActiveResources.get(key);
Resources existing = wr != null ? wr.get() : null;
if (existing != null && existing.getAssets().isUpToDate()) {
r.getAssets().close();
return existing;
}
// 将弱引用持有，缓存
mActiveResources.put(key, new WeakReference<>(r));
if (DEBUG) Slog.v(TAG, "mActiveResources.size()=" + mActiveResources.size());
return r;
}
}
接下来不考虑缓存的情况 , 我们跟踪new Resource的情况
public Resources(AssetManager assets, DisplayMetrics metrics, Configuration config,
CompatibilityInfo compatInfo) {
mAssets = assets;
mMetrics.setToDefaults();
if (compatInfo != null) {
mCompatibilityInfo = compatInfo;
}
updateConfiguration(config, metrics);
assets.ensureStringBlocks();
}
- packageInfo.getResources(mainThread)→
- mainThread.getTopLevelResources() →
- getTopLevelResources中, 已APK的路径作为key，首先查找是否存在弱引用，如不存在，直接创建Resource，创建过程必须要有AssetManager，DisplayMetrics，Configuration
AssetManager的创建是通过直接实例化对象调用了一个addAssetPath(path)方法把应用的apk路径添加到AssetManager，addAssetPath()方法请看源码解释。
创建好Resource之后会再去缓存中找Resource如果没有，那么则会创建Resource并将其缓存。 new Resources(assets, dm, config, compatInfo) 具体请看6.0源码240han。
至此，我们已经大致了解了Resource的查找过程。
3 Demo时间
在getTopLevelResources的方法中，传入了apk的路径，这个路径不止作为key来缓存Resource实例，而且是AssetsManager的初始化参数，进而将AssetsManager传入Resource的构造方法后，才完成了Resource的构造。那么，如果我们将其他akp的路径传进来，会不会就可以在我们的App里面使用其他apk的资源了？
需求: 点击按钮改变ImageView的显示图片, 且图片不在本App
首先,创建一个新的moudle, 只放入一张图片在drawable里 , 编译打包,名字随便改 我这里写test.apk
然后我们模拟Resource的创建方法
btn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
try {
//获取已创建好的Resource实例
Resources superResource = getResources();
//AssetsManager创建实例, 可以设置加载目标apk
AssetManager manager = AssetManager.class.newInstance();
//添加资源包
Method method = AssetManager.class.getDeclaredMethod("addAssetPath", String.class);
String skinPath = Environment.getExternalStorageDirectory().getAbsolutePath() + File.separator + "test.apk";
//反射执行方法
method.invoke(manager, skinPath);
Resources resources = new Resources(manager, superResource.getDisplayMetrics(), superResource.getConfiguration());
//第三个参数为包名
int drawableID2 = resources.getIdentifier("ic_test", "drawable", "ziye.skinplugin");
Drawable drawable = resources.getDrawable(drawableID2);
iv.setImageDrawable(drawable);
} catch (InstantiationException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (NoSuchMethodException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
}
}
});
AssetsManager都带有@hide标记，无法直接调用，这里用了反射
image
